Phencyclidine produces behavioral excitation through a dopamine independent mechanism in the nucleus accumbens, probably by blocking the excitatory amino acid NMDA receptor. Motor asymmetries produced by phencyclidine are caused by activation of pathways projecting caudally from the substantia nigra. Opiate-induced suppression of immune function appears to be mediated, at least in part, through the periaqueductal gray matter. The periaqueductal gray matter in general seems to be an important region of the brain for regulating the immune function. Nicotine produces behavioral excitation by activating nigrostriatal and mesolimbic dopamine pathways. Locomotor activation produced by amphetamine and cocaine in the nucleus accumbens is mediated through activation of a variety of efferent pathways projecting to the striatal fundus, ventral pallidum, zona reticulata, and the medial and lateral olivary complex. Kappa opiate receptors modulate locomotor behaviors as well as variety of homeostatic mechanisms. The depressant actions of kappa opiate agonists are not produced through the opiate receptor but appear to be nonspecific.